1. Field of the Invention
The present invention relates to a method of manufacturing a forming die such as a cold forging die, and more particularly, to a method of manufacturing a forming die that is prevented from cracking under plastic strains and tensile stresses which are applied when a material to be formed is forced into the die.
2. Description of the Related Art
It has been known that a forming die such as a cold forging die tends to develop cracks when a material to be formed is forced into the forming die. According to one theory, plastic strains produced in the forming die by the forced material are considered to be responsible for those cracks developed in the forming die. Another theory indicates that tensile stresses produced in the forming die by the forced material cause the cracks.
Since no established ideas are available for determining the cause of cracks in forming dies, some empirical trial-and-error approaches have been relied upon to prevent forming dies from cracking in use. Specifically, it has been customary to calculate tensile stresses applied to forming dies, design a forming die so that such tensile stresses will not reach fracture stresses, and, if cracks are developed in the designed forming die when it is used to actually form a material, redesign a forming die based on the experience in the design efforts.
However, the conventional procedure dictates a large expenditure of time and cost for changing designs and modifying forming dies, and is unable to fabricate uniform forming dies due to quality control instability.
Japanese laid-open patent publication No. 2-151338 discloses a forming die reinforced with a first ring held against the forming die under radial pressing forces and a second ring held against the first ring under radial pressing forces. While the second ring is being prestressed, the first ring is fitted into the second ring, thereby forming a ring assembly, and then the forming die is fitted into the ring assembly, so that the forming die is contracted in the ring assembly.
The above publication shows the application of compressive stresses radially inwardly to the forming die, but fails to clearly indicate an amount of interference between the forming die and the ring assembly and a position where such an amount of interference is to be introduced. Actually, therefore, a forming die needs to be designed according to trial-and-error attempts.